In a recent op-ed piece at the NY Times, Marcelo Aizen and Lawrence Harder write that, despite honeybee hive collapse disorder, the number of domesticated honeybee hives is actually growing worldwide, but can't keep up with our demand for what they term "luxury foods" such as almonds and avocados. They say the real concern should be our wild, native bees, whose habitats we keep destroying, in part to grow those favored crops.
That's why I think that all gardens, even (or especially) vegetable gardens, should have at least one wildish area planted to native flowers such as goldenrod, purple coneflower, beebalm, ironweed, Joe Pye weed and so forth. Bees also love "garden" plants such as Russian sage and oregano. This area should be kind of "messy," shouldn't be tilled, and should, if possible, include a pile of sand, and a small stack of dead branches. And no insecticides should be used, for obvious reasons.
Many of our native bees are solitary, and don't make hives. They live in burrows underground or in holes in wood, hence the sand and branches. An overly clean garden provides pollen, but won't persuade the bees to settle and "increase through the generations." I once saw a talk by Dr. Alan Molumby, a local entomologist, who said that the famed Lurie garden downtown is great for pollen, not so great for habitat: too clean. You'd think that the country would be better for wild bees than urban areas, but remarkably, our Chicago Wilderness is better for wild bees than many rural areas devoted to industrial farming of corn and soy. We have more different kinds of flowering plants, in vacant lots and our gardens.
For more information, The Xerces Society, devoted to invertebrate conservation, has great resources for the home gardener as well as the organic farmer. Their pollinator conservation resource center has plant lists for different regions, as well as how to provide nests and nesting materials.
Flowering Plants that Native Bees Love